When 3DR was making the solo, they decided to re-invent the wheel for a lot of things. Thing that ArduCopter already does, they wanted to do themselves to build upon and make little customizations. They created a "smart shot" called Return to Home (RTH). It is scripted in python and operates the copter in guided mode just like all the other smart shots. It rises up, flies over home, and lands. In their rush to release, combined with the ESC slew rate mess, combined with giving up completely and firing everyone, it was left not in a very good state. Nothing fancy was ever added to it, they never finished the custom land mode, and it had some nasty bugs in it. As many have noticed, it also locks out some controls.
It works ok on a stock solo. The bugs a kinda masked but they're present if you look hard. Use ArduCopter master without the slew rate limiting and it gets really ugly. An in any case, it is a huge point of failure. These smart shots are vulnerable to failure and hiccups. Using something buggy and unnecessary like that as a failsafe is not a good idea. ArduCopter's native RTL mode works better to begin with, is rock solid stable, has no bugs, and nothing to go wrong.